The heart of a computer is a magnetic hard disk drive (HDD) which typically includes a rotating magnetic disk, a slider that has read and write heads, a suspension arm above the rotating disk and an actuator arm that swings the suspension arm to place the read and/or write heads over selected circular tracks on the rotating disk. The suspension arm biases the slider into contact with the surface of the disk when the disk is not rotating but, when the disk rotates, air is swirled by the rotating disk adjacent a media-facing surface of the slider causing the slider to ride on an air bearing a slight distance from the surface of the rotating disk. When the slider rides on the air bearing the write and read heads are employed for writing magnetic impressions to and reading magnetic signal fields from the rotating disk. The read and write heads are connected to processing circuitry that operates according to a computer program to implement the writing and reading functions.
The volume of information processing in the information age is increasing rapidly. In particular, it is desired that HDDs be able to store more information in their limited area and volume. A technical approach to this desire is to increase the capacity by increasing the recording density of the HDD. To achieve higher recording density, further miniaturization of recording bits is effective, which in turn typically requires the design of smaller and smaller components.
One attempt at miniaturizing components has led to the use of read heads that employ a magnetoresistance effect film in which a sensing current flows in a direction perpendicular to the plane of the film. These read heads may utilize a tunneling magnetoresistance (TMR) film or a current-perpendicular-to-plane (CPP)-type giant magnetoresistance (GMR) film. Such a film comprises an insulating layer, in the case of a TMR film, or a metallic layer, in the case of a GMR film, that is sandwiched by two ferromagnetic layers. Typically, in the case of the lower ferromagnetic layer, the direction of magnetization is fixed in one direction (e.g., a fixed layer) by a coupling magnetic field having an antiferromagnetic (AFM) layer, while a direction of magnetization of the upper ferromagnetic layer (e.g., a free layer) is rotated by a leakage magnetic field produced from the recording medium. In the case of a TMR film or a GMR film, the magnetoresistance of the film is altered by the relative angle of magnetization of the two ferromagnetic layers. The read output is therefore obtained by sensing a change of the film resistance produced by rotation of the magnetization of the free layer in response to the direction of the leakage magnetic field from each of the recording bits on the recording medium.
The size of the magnetoresistance effect film, e.g., the TMR film or the GMR film, that is exposed at a media-facing surface of the read head must therefore be selected in accordance with a surface recording density of the recording medium. The width of the free layer in the film surface direction that is exposed at the media-facing surface is called the track width and determines read resolution in the track direction.
Also, the magnetoresistance effect film is typically sandwiched, above and below, by soft magnetic shields. A distance (substantially determined by the thickness of the magnetoresistance effect film) between these upper and lower shields is termed the gap length and determines the read resolution in the bit direction. Also, the length of the free layer of the magnetoresistance effect film in the film surface height direction as seen from the media-facing surface is selected to be a suitable length, taking into account the aspect ratio of the track width and the resistance of the magnetoresistance effect film. Basically, these dimensions have to be made progressively smaller as the film recording density of the recording medium is increased.
In addition, magnetic domain control films are typically arranged on the left and right sides, in the track width direction, of the magnetoresistance effect film. A biasing magnetic field, of a suitable magnitude, is applied in the track width direction to the free layer of the magnetoresistance effect film by the magnetic domain control films, in order to ensure linearity of the read output. Previously, a hard magnetic film was employed for the magnetic domain control films, but, in recent years, methods employing a soft magnetic film have been developed. A biasing magnetic field is applied to the free layer by conferring anisotropy on the soft magnetic film using an AFM layer that is arranged on the upper shield layer. When a soft magnetic film is employed, compared with when a hard magnetic film is employed, the biasing magnetic field may be made stronger and the soft magnetic film functions as a shield, so an improvement in read resolution performance in the track direction may be achieved. Consequently, in recent years, there has been a trend towards applying biasing magnetic field using a soft magnetic film.
Recently, with increases in the surface recording density of hard disk drive (HDD) media, the size of the gap length and the track width of the magnetoresistance effect film, as described above, has been reduced to a few tens of nanometers, so the volume of the magnetoresistance effect film has become extremely small. Consequently, the volume of the fixed layer, including the AFM layer of the magnetoresistance effect film, has also become extremely small, leading in recent years to the lowering of the stability of the fixed layer. Consequently, in recent years, read heads which have a fixed layer with as large a volume as possible have been developed in order to ensure stability of the fixed layer.
This structure is formed by using a partial milling process so that a fixed layer remains in the element track direction and height direction. The remaining volume of the fixed layer is greatly increased by using a construction in which the fixed layer is left behind in both the track direction and the height direction, so improved stability can be expected.
However, if such a construction is adopted, in which the fixed layer remains, the space available for providing the magnetic domain control film for applying the biasing magnetic field becomes small. Also, the relative position of the magnetic domain control film with respect to the free layer is elevated. Consequently, when such a construction in which the fixed layer remains behind was adopted a problem arose where the effective biasing magnetic field applied to the free layer becomes insufficient.
This problem is severe when a hard magnetic film of complicated film structure requiring an underlayer film is employed for the magnetic domain control film; however, when a soft magnetic film is employed for the magnetic domain control film, the biasing magnetic field is increased compared with the construction where a hard magnetic film is employed. Unfortunately, there is still the problem the effective biasing magnetic field applied to the free layer being insufficient.